Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913, Eastman Kodak Company introduced its first product specifically intended to be exposed by X-radiation (X-rays). Silver halide radiographic films account for the overwhelming majority of medical diagnostic images. It was recognized almost immediately that the high energy ionizing X-rays are potentially harmful, and ways were sought to avoid high levels of patient exposure. Radiographic films provide viewable silver images upon imagewise exposure followed by rapid access processing.
One approach still in use is to coat the silver halide emulsions useful in radiographic films on both sides of the film support. Thus, the number of X-rays that can be absorbed and used for imaging are doubled, providing higher sensitivity. Dual-coated radiographic films are sold by Eastman Kodak Company as DUPLITIZED.TM. films. Films that rely entirely upon X-radiation absorption for image capture are referred to in the art as "direct" radiographic films while those that rely on intensifying screen light emission are referred to as "indirect" radiographic films. Because the silver halide emulsions in direct radiographic elements are used to capture the X-rays directly, the silver coating coverages are generally higher than for indirect radiographic elements.
Among the "direct" radiographic films, are films most commonly used for dental intra-oral diagnostic imaging and hereafter referred to as dental films. Intra-oral dental imaging presents obvious barriers to the use of intensifying screens. There are other applications for direct radiographic films, such as in various industrial applications where X-rays are captured in imaging, but intensifying screens cannot be used for some reason.
It is the prevailing practice to process direct radiographic films for more than 3 minutes because of higher silver coverage. Such processes typically include black-and-white development, fixing, washing and drying. Films processed in this manner are then ready for viewing.
Photographic developing solutions containing a silver halide developing agent are well known in the photographic art for reducing silver halide grains containing a latent image to yield a developed photographic image. Many useful developing agents are known in the art, with hydroquinone and similar dihydroxybenzene compounds and ascorbic acid (and derivatives) being some of the most common. Such solutions generally contain other components such as sulfites as antioxidants, buffers, antifoggants, halides and hardeners. A workable pH for such solutions is usually in the range of from about 10 to about 11, depending upon the developing agent and other solution components.
Fixing solutions for radiographic films are also well known and include one or more fixing agents, of which thiosulfates are most common. Such solutions also generally include sulfites as antioxidants, and hardeners (such as aluminum salts), and a buffer (such as acetate), and have a functional pH range of from about 4 to about 5.5.
Sulfite ion has long been known as a preservative in photographic developing compositions. Thus, it has become a universal preservative for black-and-white developing agents by stabilizing and maintaining uniform activity. In addition, high concentrations of sulfite ions are known to provide considerable protection from aerial oxidation by reducing the solubility of oxygen in the developing composition. High molar ratios of sulfite ion to hydroquinone type developing agents are common in black-and-white developing compositions (see for example, TABLE V below). However, high sulfite levels are often undesirable because sulfite ion can act as a weak silver complexing agent. In addition, U.S. Pat. No. 4,672,025 (Yamada et al) suggests molar ratios of 1.6:1 to 3.5:1 with molar ratios of from 1.65:1 to 2.8:1 being preferred.
Borates are known as buffering agents in black-and-white developing compositions (see for example U.S. Pat. No. 4,810,622 of Yamada et al and U.S. Pat. No. 5,702,875 of Opitz et al), but borate ions typically have limited solubility, so they are often used in combination with other buffering agents. They also limit solution alkalinity so that higher pH is difficult to maintain using borates alone.
In high activity developing compositions used for rapid processing, the level of hydroquinone developing agent is quite high. However, this limits the molar ratio of sulfite to hydroquinone, without which in time, stability problems are encountered due to aerial oxidation and hydroquinone degradation. Increasing the sulfite level alone at this point is ineffective. Moreover, as hydroquinone activity is lost, the activity from co-developers such as 3-pyrazolidone co-developers is also prematurely diminished.
This is a need for a black-and-white developing composition that can include high hydroquinone developing agent concentration, but also have the desired stability without the need for high chemical loading with sulfites.